1. Field of the Invention
The present invention relates generally to a method of beneficially utilizing solid waste materials and, more particularly, to a method for refining and converting solid waste to liquid fuel, such as methanol.
2. Description of the Prior Art
The problem of disposing of solid waste has been increasing for many years with the growth of population and is now critical in the larger cities. The usual manner of disposing of solid waste has been to use it for land fill, to dump it into water, or it incinerate it. Suitable land fill areas are becoming increasingly hard to find. Dumping solid wastes into water presents pollution problems. Incinerating solid wastes pollutes the air with harmful gases and increases its CO.sub.2 content When rains remove excess CO.sub.2 content from the air, the acidity of rain water is increased and this may have harmful effects on plants, animal life and buildings. Moreover, incinerators require tall stacks and scrubbers to remove harmful components from the gas and these structures are quite expensive.
Numerous efforts have been made to solve solid waste problems but none have been entirely satisfactory. One proposal subjects solid wastes to dry distillation or pyrolysis at temperatures between about 900.degree. C. and about 1000.degree. C. without the injection of steam or oxygen and thereby attempts to produce crude oil. Scrubbing the gas products of this process is required and the scrubbing treatment causes pollution of water and the atmosphere.
Another proposal includes the steps of applying heat externally to a retort in which garbage is heated without internal combustion. The garbage is distilled or pyrolized at a temperature of about 400.degree. C. The resulting gas is cracked, enriched, and scrubbed to make it suitable for heating and illuminating purposes. However, the scrubbing of the gas creates pollution problems and about 30% of the carbon remains unused and is discarded with the ash.
Another proposal calls for garbage to be destructively distilled while on a traveling grate in an open system. Organic material is thermally decomposed at temperatures between about 1000.degree. F. and about 2000.degree. F. in the absence of oxygen. Some 34-36% of the starting material remains at the completion of the process. Most of the gas produced during the process is consumed in the process and the gas which is not so consumed is heavily diluted with carbon dioxide and nitrogen and is not suitable for use in the chemical industry or release to the atmosphere.
As the public concern about air pollution has increased, stack heights have been increased to affect better dispersion of pollutants. Increasing stack heights add to the cost of constructing and maintaining stacks, yet provides no solution to the underlying problem, i.e., avoiding emission in the first instance of harmful substances such as sulfur oxides, chlorine gases, phosphor oxides, etc.
A significant factor in air pollution is the increasing level of gaseous airborne pollutants which combine with moisture in the air to produce acids, e.g., carbon dioxide, sulfur dioxide, and compounds of chlorine and fluorine. The carbon dioxide content in some industrial districts is as high as ten times normal. Acid forming pollutants have been found in some instances to increase the acidity of rainwater from its normal pH of about 6.9 to values of 4.0. Rainwater having a pH of 5.5 or lower will destroy aquatic life and can do substantial harm to buildings, monuments, and other structures.
One proposal for removing acid forming components from exhaust gases is to scrub the entire flow of exhaust gases with water and caustic prior to discharging them through a stack. However, scrubbing the entire exhaust gas flow requires large quantities of water, which are not always available, and requires costly, large capacity scrubbing equipment. Indeed, scrubbing the entire flow of exhaust gases from some incinerators requires at least half the amount of water, by weight, of the solid wastes burned in the incinerator. Treating the large volume of scrub water needed in such a process is very costly and contributes to the impracticality of scrubbing as a total solution to the acid pollutant problem.
The problem of disposing of exhaust gases is now recognized as a major concern in industrial countries throughout the world. Dispersing emissions through the use of tall stacks is no longer regarded as an acceptable solution. Applicant's U.S. Pat. No. 3,970,524 discloses a system for gasification of solid waste materials and a method for treating the resulting gases to produce commercially usable gases in such a manner that dispersion through stacks is not necessary. Solid waste materials are initially converted into combustion gas by partial oxidation in a combustion zone with a minimum amount of oxygen and subsequently purified into various components or fractions useful in industry. Purification occurs in stages by successive condensing steps whereby contaminants and undesired components are removed and desirable components are separated into useful gaseous fractions for heating boilers, firing gas turbines, providing district heat, and synthesis of ammonia and other chemical products, including plastics, dye stuffs and pharmaceuticals. One feature of the process is the use of a multichamber gas treatment unit in which noxious gas components are condensed, sublimed or "frozen out" and thereby separated from the clean usable gas components. However, the process discharges CO.sub.2 to the atmosphere and, due to the high nitrogen content in the combustion gas, is not suitable for producing a synthesis gas which can be converted to a liquid fuel, such as methanol.